Heat exchangers are employed within the automotive industry as radiators for cooling engine coolant, condensers and evaporators for use in air conditioning systems, and heaters. In order to efficiently maximize the amount of surface area available for transferring heat between the fluid within the heat exchanger and the environment, the design of the heat exchanger is typically of a tube-and-fin type containing a multitude of tubes which thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa. Heat exchangers used in the automotive industry are often formed from aluminum alloys in order to help reduce the weight of automobiles.
Heat exchangers are increasingly being formed by a brazing operation in which the individual components of the heat exchanger are permanently joined together with a brazing alloy. Generally, brazed heat exchangers are lower in weight and are better able to radiate heat as compared to heat exchangers formed by known mechanical assembly techniques. An example of a brazed heat exchanger is of the serpentine tube-and-center (STC) type, which is characterized by one or more serpentine-shaped tubes that are brazed to a number of high surface area finned centers, with an inlet and outlet being located at opposite ends of the tube or tubes. Another type of heat exchanger is the headered tube-and-center (HTC) type, which utilizes a number of parallel tubes which are brazed to and between a pair of headers, with finned centers being brazed between each adjacent pair of tubes. Conventionally, headered tube-and-center type heat exchangers have been constructed by inserting the parallel tubes into apertures formed in each of an opposing pair of headers. A finned center is then positioned between each adjacent pair of parallel tubes. Tanks are formed at each header so as to be in fluidic communication with the tubes through the apertures. The tanks include ports which serve as an inlet and outlet to the heat exchanger.
The above individual components are fixtured together before undergoing a furnace brazing operation that forms numerous brazements which permanently join the components to form a heat exchanger assembly. Generally, the brazements are achieved by forming the headers and the finned centers from an aluminum alloy brazing stock material composed of an aluminum-base brazing alloy layer which is clad on at least one surface of an aluminum alloy core. Typically, the brazing alloy is an aluminum-silicon eutectic alloy, such as AA 4045, AA 4047 and AA 4343 aluminum alloys (AA being the designation given by the Aluminum Association), which has a melting point that is lower than the core alloy, which is often AA 3003. The brazing operation involves raising the temperature of the assembly such that only the clad layers of brazing alloy melt during the brazing operation. Upon melting, the brazing alloy flows toward the desired joint regions and, upon cooling, solidifies to form the brazements.
The brazing operation for a headered tube-and-center heat exchanger is particularly complicated by the numerous brazements required for each tube, each of which must be brazed to both headers and its corresponding finned centers during a single brazing operation. To destroy and remove the aluminum oxide layer which is inherently present on the brazing stock material, and thus enhance the brazeability of the brazing and core alloys, the assembly or its individual components are generally sprayed with or dipped into a flux mixture composed of a water-insoluble flux material suspended in a liquid medium. For furnace brazing, more flux is required at the tube-to-header joint than the tube-to-fin joint so as to ensure that a fluid-tight seal is formed. In order to satisfy the flux requirements for both the tube-to-header and tube-to-fin joints, conventional flux mixtures typically consist of about 5 to about 25 volume percent of flux material suspended in water. The entire assembly is coated with the flux mixture and then dried to evaporate the water, leaving only the powdery flux material on the external surfaces of the assembly. Removal of the water is necessary so as to deter oxidation of the aluminum alloys during brazing, which would otherwise be detrimental to the brazeability of the heat exchanger. To further minimize the presence of moisture during brazing, the brazing atmosphere is typically composed of cryogenic nitrogen which is maintained at a dewpoint of no more than about -43.degree. C. (-45.degree. F.), with a free oxygen level of 100 parts per million (ppm) or less.
The basic form of flux material used in flux mixtures has been potassium fluoroaluminate complexes, consisting of a mixture of potassium aluminum fluoride (K.sub.3 AlF.sub.6) and potassium tetrafluoroaluminate (KAlF.sub.4), as disclosed in U.S. Pat. No. 3,951,328 to Wallace et al. and U.S. Pat. No. 3,971,501 to Cooke. These flux materials are molten at brazing temperatures, and upon cooling leave a residue forming a thin, uniform ceramic film. However, a shortcoming of these conventional flux mixtures is that, at brazing temperatures, potassium aluminum fluoride reacts with water inherently present in the potassium aluminum fluoride particles to form potassium fluoride (KF) and hydrogen fluoride (HF), both of which are extremely toxic and highly corrosive to the interior of the brazing furnace. The reaction is as follows: EQU 2K.sub.3 AlF.sub.6 +3H.sub.2 O+6KF+Al.sub.2 O.sub.3 +6HF
Accordingly, the elimination of water vapor within the brazing furnace is an extremely important consideration when using a flux material which contains potassium aluminum fluoride. However, it is impossible to prevent the presence of water vapor during the brazing operation in that water is inherently present in the potassium aluminum fluoride particles. In addition, these flux materials must be suspended in water in order to be applied using spraying and dipping methods. Consequently, even after drying, additional moisture will be contributed to the brazing atmosphere. In addition, spraying and dipping methods result in the deposition of flux on surfaces of the heat exchanger components which do not serve as braze joint areas and thus do not require flux. The presence of this excess flux material further promotes the creation of the undesirable potassium fluoride and hydrogen fluoride.
Known methods by which aluminum alloy brazing stock materials are produced also contribute to the use and/or presence of excess flux. For example, stock materials are often formed by: forming the brazing alloy as a foil which is brazed to the aluminum alloy core; or coating the aluminum alloy core with a molten brazing alloy. As a result, two fluxing operations are required--the first to adhere the brazing alloy to the aluminum alloy core, and a second to braze the tubes to the headers and finned centers during the braze furnace operation. In addition to promoting the creation of a corrosive byproduct, the use of two fluxing operations is disadvantageous in that the application and removal of the excess flux material, as well as the necessary effluent control procedures required to treat the waste water generated by flux removal, add costs to the final assembly.
U.S. Pat. No. 5,180,098 to Halstead et al. teaches a concentrated flux mixture which allows the amount of flux mixture used to be reduced. In effect, the flux mixture also reduces the amount of potassium fluoride and hydrogen fluoride produced by minimizing the amount of flux mixture which must be used, and limiting the amount of water required in the flux mixture to suspend the flux material. However, as with most conventional flux mixtures, the flux mixture taught by Halstead et al. contains potassium fluoaluminate particles (i.e., both potassium aluminum fluoride and potassium tetrafluoroaluminate), as taught by U.S. Pat. No. 3,951,328 to Wallace et al. Accordingly, the flux mixture produces hydrogen fluoride and potassium fluoride as a corrosive byproducts due to the presence of potassium aluminum fluoride. Another disadvantage with known flux mixtures, including that taught by Halstead et al., is the tendency for the constituents of such mixtures to separate from suspension over time, typically in as little as 24 hours. Within less than a week, a hard residue forms at the bottom of the storage container which cannot be put back into suspension, rendering the flux mixture useless.
U.S. Pat. No. 5,251,374 to Halstead et al. teach a novel flux composition in which the flux material is a minimum of 96 percent potassium tetrafluoroaluminate, and is free of potassium aluminum fluoride. Consequently, such a flux composition does not produce hydrogen fluoride during brazing. However, the flux composition taught by Halstead et al. is susceptible to separating over time, which significantly limits the shelf life of the composition.
From the above, it is apparent that the brazing operation for a heat exchanger, and particularly a headered tube-and-center heat exchanger, is complicated by the numerous brazements which must be formed during a single brazing operation. Furthermore, conventional brazing operations undesirably make use of flux mixtures which, in an effort to suitably form the numerous required brazements, results in the presence of excess flux, and produce corrosive byproducts which shorten the service life of the brazing furnace. Finally, conventional flux mixtures have an extremely short shelf life, which further complicates the manufacture of brazed heat exchangers.
Accordingly, it would be desirable to provide a flux mixture which does not produce corrosive byproducts during the brazing operation, while also having an extended shelf life. In addition, it would be desirable if such a flux mixture made possible a method for forming a heat exchanger in which the amount of flux mixture required was significantly reduced, and in which the use of a clad brazing stock material alloy could be reduced or eliminated in order to further limit the use of the flux mixture in the manufacture of brazed heat exchangers.